The Q-switch technique is used in high-power pulsed lasers to temporarily block the amplification medium while exciting it beyond the degree ordinarily needed for continuous oscillations. A Q-switch is an optical switch for blocking a light transmission path. The blocking or shutter action inhibits laser action while the number of excited atoms increases. Once the energy stored by the excited atoms reaches a level necessary to sustain the required high power pulse, the shutter is abruptly "opened" and the stored energy is released in a brief time as a high power pulse. This fast rise time opening of the Q-switch is a key requirement.
Various materials have been used as pulsed laser amplifying media. For example, neodymium ions in crystals such as yttrium aluminum garnet (Nd:YAG) have been successfully used in an optically pumped solid-state laser providing peak output powers on the order of several megawatts in a pulse lasting 20 nanoseconds. Although some lasers are three-level devices or media, i.e., after excitation from a ground state to a high-energy state, atoms quickly relax to an emitting state, the Nd:YAG laser uses a fourth level as a terminal level for the laser transition. Relaxation to the ground state does not produce any optical radiation. Compared to a three-level laser, population inversion is more easily achieved and requires relatively low pumping light intensity.
In the past, Q-switched solid-state lasers comprised individual elements, e.g., mirrors, rod, and fast Q-switch, each of which had to be individually mounted and optically aligned. For many applications, accurate optical alignment had to be maintained over a wide range of temperature, shock, and vibration conditions. Many individual parts had to be made which added to the cost.
An improvement to this situation was the passive Q-switch laser cartridge. A cartridge unit was constructed in which the elements were bonded together in one integral structure. A problem with the passive Q-switch laser cartridge, however, is that it is only applicable to the case of a passive Q-switch element. Although passive Q-switch elements are known for the ruby and neodymium laser wavelengths (0.6943 and 1.06 microns, respectively), suitable materials have not been found for other wavelengths of interest such as 1.54, 1.73, 2.06 microns, etc. These wavelengths represent outputs from other solid-state lasers for which a small integral laser "cartridge-like" approach would be useful. However, a passive Q-switch does not exist at these wavelengths, and many other wavelengths of interest, and an active Q-switch element must be employed in these cases.
An acousto-optical active Q-switch device consists of a block of transparent photoelastic material, such as fused quartz. An ultrasonic transducer is bonded to one face, an absorbing material bonded to an opposite face and a radio frequency (RF) power source is provided for driving the ultrasonic transducer to generate a beam of acoustic waves in the photoelastic block. The wavelength of the sound waves is selected to be comparable in magnitude to the diameter of the light beam. The interaction of the light beam emitted by the laser medium with the acoustic waves in the photoelastic block results in Bragg-effect scattering of the light beam thus "spoiling" the resonant quality (Q) of the optical cavity. Laser oscillations are inhibited and the population of excited atoms increases until the stored energy reaches a level sufficient to sustain the desired pulse. Once the level is reached, the ultrasonic beam is turned off, the Q of the optical cavity is restored and the stored energy is suddenly released resulting in a high-peak power laser output pulse.
U.S. Pat. No. 3,464,027, LASER MODULATION BY FOCUSED ACOUSTIC ENERGY, to DeMaria, patented Aug. 26, 1969, discloses a modulated laser wherein the transducer is formed with a hole therethrough adapted to receive a laser rod. The thick, low frequency transducer, in this case, is bonded to the rod, which is used as both an acoustic medium and a laser amplifying medium for low frequency modulation.
U.S. Pat. No. 3,828,276, HIGH EFFICIENCY ACOUSTO-OPTICAL Q-SWITCH, to Cohen, patented Aug. 6, 1974, discloses a laser Q-switch of the type in which optical alignment of each element is required. See also U.S. Pat. No. 4,276,519.
U.S. Pat. No. 3,435,372, ULTRASONIC MODULATOR HAVING A CYLINDRICAL TRANSDUCER, to Aas et al, patented Mar. 25, 1969, discloses a tubular cylindrical transducer placed around and concentric with a laser rod and filled with a suitable liquid. The transducer is driven at the radial resonance frequency of the rod. The ultrasonic energy produced by the transducer is coupled through the liquid and focuses in the laser rod to produce a radial resonance in the laser rod. This radial standing wave spoils the "Q" of the laser optical cavity, and the laser energy is gated at a frequency directly proportional to the frequency of the low frequency standing wave.
U.S. Pat. No. 3,544,916, MULTIPLE FREQUENCY ULTRASONIC CONTROL FOR LASERS, to Angelbeck, patented Dec. 1, 1970, discloses a pair of low frequency cylindrical transducers directly bonded to a laser rod, each transducer driven at an ultrasonic frequency slightly different by an amount equal to the modulation frequency desired for the laser. The transducers are positioned immediately adjacent to one another.